Anomalous ionospheric behaviors cause spatially different refraction effects on the signals of Global Navigation Satellite System (GNSS). This can pose potential integrity threats to Ground-Based Augmentation System (GBAS) users. Very rare ionospheric threats, which are not monitored by range-domain monitors within GBAS ground facility, are mitigated via position-domain geometry screening by inflating some of integrity parameters under the worst-case ionospheric conditions. However, this underlying assumption with a very small probability reduces usable satellite subset geometries in process of inflation, and leads to availability loss of GBAS. This paper utilizes Space-Based Augmentation System (SBAS) Grid Ionospheric Vertical Error (GIVE) to mitigate severe ionospheric threats, and improves the CAT-I GBAS availability in the Korean region. Ionospheric observation data are obtained from 5 South Korean GNSS reference stations for computation of GIVEs, and the GIVEs are used for determination of execution of position-domain geometry screening. In addition, CAT-I GBAS availability is evaluated by computing a 24-hour average of instantaneous availability. The result demonstrates that the use of GIVEs reduces not only unnecessary inflation but also availability degradation. This benefit of using data external to GBAS can reduce the conservatism of worst-case ionospheric assumption for implementation of GBAS concerning availability loss in the future.

Keywords: GBAS, SBAS GIVE, integrity, availability, ionospheric anomaly mitigation